1. Field of the Invention
The present invention relates to continuously variable transmissions (“CVTs”) and to methods of control thereof.
The invention is especially, but not exclusively, applicable to transmissions in motor vehicles.
2. Background Art
CVTs conventionally use some form of ratio varying unit (“variator”) to provide the required continuous variation of transmission ratio, the variator being coupled between transmission input and output via gearing and one or more clutches. The present invention has been developed in connection with transmissions using variators of toroidal-race rolling-traction type but is potentially applicable to certain other types of variator.
In a variator drive is transmitted by frictional engagement between moving parts, these parts being biased into engagement with each other. Thus for example in the toroidal-race rolling-traction type variator drive-is transmitted from one rotating disc to another by a roller, or more typically a set of rollers, sandwiched between the discs. A biasing force is applied (eg by urging one disc toward the other, although the rollers themselves may instead be biased toward the discs) to create pressure between the roller and discs and so enable transmission of drive. In fact the rollers and discs are in this case separated by a thin film of “traction fluid” and the required friction is a result of shear in the film.
The torque which can be transmitted by the variator is limited inter alia as a function of the biasing force. Furthermore the biasing force is typically varied in operation since large forces are required when large torque is being handled, but to sustain such a large biasing force would reduce energy efficiency and component lifetime.
One of the long recognised requirements in designing a variator is ensuring that a mismatch does not arise between the biasing force and the torque being handled, since excessive torque loading on the variator, in relation to the biasing force, risks creating an unacceptable level of slip between the variator's moving components and consequent variator failure or damage.